This invention relates to a vision prosthesis, and in particular, to prosthetic lenses.
In the course of daily life, one typically regards objects located at different distances from the eye. To selectively focus on such objects, the focal length of the eye""s lens must change. In a healthy eye, this is achieved through the contraction of a ciliary muscle that is mechanically coupled to the lens. To the extent that the ciliary muscle contracts, it deforms the lens. This deformation changes the focal length of the lens. By selectively deforming the lens in this manner, it becomes possible to focus on objects that are at different distances from the eye. This process of selectively focusing on objects at different distances is referred to as xe2x80x9caccommodationxe2x80x9d.
As a person ages, the lens loses plasticity. As a result, it becomes increasingly difficult to deform the lens sufficiently to focus on objects at different distances. To compensate for this loss of function, it is necessary to provide different optical corrections for focusing on objects at different distances.
One approach to applying different optical corrections is to carry different pairs of glasses and to swap glasses as the need arises. For example, one might carry reading glasses for reading and a separate pair of distance glasses for driving. This is inconvenient both because of the need to carry more than one pair of glasses and because of the need to swap glasses frequently.
Bifocal lenses assist accommodation by integrating two different optical corrections onto the same lens. The lower part of the lens is ground to provide a correction suitable for reading or other close-up work while the remainder of the lens is ground to provide a correction for distance vision. To regard an object, a wearer of a bifocal lens need only maneuver the head so that rays extending between the object-of-regard and the pupil pass through that portion of the bifocal lens having an optical correction appropriate for the range to that object.
The concept of a bifocal lens, in which different optical corrections are integrated into the same lens, has been generalized to include trifocal lenses, in which three different optical corrections are integrated into the same lens, and continuous gradient lenses in which a continuum of optical corrections are integrated into the same lens. However, just as in the case of bifocal lenses, optical correction for different ranges of distance using these multifocal lenses relies extensively on relative motion between the pupil and the lens.
Once a lens is implanted in the eye, the lens and the pupil move together as a unit. Thus, no matter how the patient""s head is tilted, rays extending between the object-of-regard and the pupil cannot be made to pass through a selected portion of the implanted lens. As a result, multifocal lenses are generally unsuitable for intraocular implantation because once the lens is implanted into the eye, there can be no longer be relative motion between the lens and the pupil.
A lens suitable for intraocular implantation is therefore generally restricted to being a single focus lens. Such a lens can provide optical correction for only a single range of distances. A patient who has had such a lens implanted into the eye must therefore continue to wear glasses to provide optical corrections for those distances that are not accommodated by the intraocular lens.
The invention provides a vision prosthesis for restoring a patient""s ability to focus on objects at different distances. The vision prosthesis includes a lens whose focal length can automatically be changed, and a rangefinder coupled to that lens for estimating the range to an object that the patient wishes to focus on.
In one embodiment, the variable-focus lens of the vision prosthesis has an index of refraction that varies in response to a focusing stimulus. An actuator in communication with the lens provides the necessary focusing stimulus on the basis of a range estimate from the rangefinder. A controller coupled to the rangefinder and to the actuator causes the actuator to generate a focusing stimulus on the basis of this range estimate.
Because it is the index of refraction that is changed, the vision prosthesis provides control over the focal length of the lens without the need to mechanically move the lens or any portions thereof. The vision prosthesis thus provides a lens of variable focal length with no moving parts and without the complexity and excessive power consumption associated with a moveable system.
The lens of the vision prosthesis can be adapted for implantation in an eye of a phakic or an aphakic human patient. Alternatively, the lens, and its associated electronics, can be worn outside the patient on, for example, an eyeglass frame.
When implanted in the eye, the lens can be disposed at a variety of locations, such as the anterior chamber, the posterior chamber, the lens bag, or the cornea. To ease the implantation process and to minimize the extent of the incision required, the lens can be a foldable lens having a tendency to spring back into an unfolded state.
In one embodiment of the vision prosthesis, the lens includes a chamber containing a nematic liquid crystal or other material that has a changeable index of refraction. A nematic liquid crystal has an index of refraction that changes in response to an applied electromagnetic field. This change in the index of refraction results in a change in the focal length of the lens.
The actuator for the lens can include a variable voltage source and one or more electrodes coupled to both the variable voltage source and the lens. Alternatively, the actuator can include a variable current source and one or more coils coupled to the variable current source and to the lens. In either case, the actuator generates a field, an electric field in the former case and a magnetic field in the latter case, that can interact with the nematic liquid crystal to selectively alter its index of refraction.
The index of refraction of the lens need not be spatially uniform. By providing a plurality of actuating elements coupled to different local regions of the lens, the index of refraction can be varied at those local regions. This enables the lens to have an effective optical shape that is largely independent of its physical shape. A convex lens can be created, for example, by applying a stronger electric field to the central portion of a planar chamber filled with nematic liquid crystal than to the periphery. This changes the index of refraction at the center more than at the periphery. A lens having a spatially non-uniform index of refraction can be implemented by providing a plurality of electrodes disposed at different portions of the lens. In one aspect of the invention, these electrodes are concentric electrodes. In such a case, the index of refraction can be made a function of distance from the center of the lens.
In an alternative embodiment, the index of refraction can be made a function of more than one spatial variable. For example, the electrodes can be distributed in a two-dimensional grid on the surface of the lens. Such a grid can be a polar grid or a rectilinear grid. Its primary function would be to correct wavefront aberrations present in the eye due to abnormalities in the cornea, the lens, and the ocular media.
An advantage of a lens having planar chamber as described above is that such a lens can be made thin enough to be implanted in very small spaces within the eye. For example, a lens in which first and second planar sides are separated by a gap smaller than the separation between the lens bag in an eye and the iris in the eye can be implanted in the posterior chamber of the eye.
In some cases, it may not be possible to vary the index of refraction sufficiently to correct the patient""s vision. In such cases, the lens can include one or more lens elements that can be moved so as to bring an image into focus. Such a lens also includes a motor to move the lens elements.
Alternatively, the lens can have a baseline curvature and also be filled with nematic crystal or a material having an index of refraction that can be changed. The baseline curvature can be used to perform a gross correction that can be fine-tuned by locally varying the index of refraction of the lens material.
In one embodiment of the vision prosthesis, the rangefinder includes a transducer for detecting a stimulus from an anatomic structure in an eye, the stimulus being indicative of a range to the object-of-regard. The transducer can be a pressure transducer for detecting contraction of a muscle, such as a piezoelectric element that generates a voltage in response to contraction of the muscle. Alternatively, the transducer can be an electromyograph for detecting electrical activity associated with contraction of the muscle.
The stimulus detected by the transducer can come from the activities or states of one or more anatomical structures within the eye. These activities or states include: contraction of a ciliary muscle, tension in a zonule, mechanical disturbance of a lens bag, contraction of a rectus muscle, and dilation of an iris.
The rangefinder of the vision prosthesis does not, however, have to rely on the operation of any structure in eye to estimate a distance to an object. For example, the rangefinder can also include an autofocus system. One example of an autofocus system includes: an infrared transmitter for illuminating an object with an infrared beam; an infrared receiver for receiving a reflected beam from the object, and a processor coupled to the infrared receiver for estimating a range to the object on the basis of the reflected beam. However, other autofocusing systems can readily be adapted for the use in the vision prosthesis.
To assist the autofocus system in achieving and maintaining focus, it is often desirable to include a feedback loop coupled to the autofocus system. One example of a feedback loop includes first and second lenslets posterior to the lens. Each lenslet is in optical communication with an associated photodetector posterior to that lenslet. The distance between the lenslet and its associated photodetector is between the focal lengths of the two lenslets.
Regardless of the type of rangefinder, it is useful to provide an optional manual focusing control for enabling a patient to fine tune focusing of the lens. A manual focusing control enables the patient to correct compensate for minor inaccuracies in the signal provided by the automatic focusing system. With a manual focusing control, the rangefinder can in fact be dispensed with. Thus, in yet another embodiment of the invention, the apparatus includes a manual focusing control instead of a rangefinder.
These and other features and advantages of the invention will be apparent from the following detailed description and the accompanying figures, in which: